Work out telescope magnification instantly with clear inputs, formula shown and shareable results.
Magnification is objective focal length divided by eyepiece focal length: a 1200 mm scope with a 10 mm eyepiece gives 120x. The exit pupil, aperture divided by magnification, is 1.25 mm here - comfortable for planetary work. Maximum useful magnification is roughly twice the aperture in millimetres, beyond which the image only grows blurrier.
Magnification
M = objective focal length / eyepiece focal length
Exit pupil
exit pupil = aperture / magnification
Because diffraction sets the resolution. Past about 2x the aperture in millimetres you magnify blur, not detail.
About 2-3 mm for general viewing, 5-7 mm for faint deep-sky objects and under 1 mm only for very high power planetary work.